Heat Recovery System and Method

ABSTRACT

The present techniques are directed to a heat recovery system, such as a waste heat recovery system (WHRU) that receives and passes a vapor across a heat exchanger to transfer heat from the vapor to a heating medium in the heat exchanger. The vapor may be an exhaust gas from a source outside of the heat recovery system. The heat recovery system includes a collection system to deinventory the heating medium from the heat exchanger during abnormal operation of the heat recovery system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. patent applicationNo. 62/031,720 filed Jul. 31, 2014 entitled HEAT RECOVERY SYSTEM ANDMETHOD, the entirety of which is incorporated by reference herein.

FIELD

The present techniques relate generally to heat recovery such as wasteheat recovery, and more particularly to a heat recovery system having aheating-medium collection system for abnormal operation to reducethermal degradation of the heating medium.

BACKGROUND

This section is intended to introduce various aspects of the art, whichmay be associated with exemplary embodiments of the present techniques.This discussion is believed to assist in providing a framework tofacilitate a better understanding of particular aspects of the presenttechniques. Accordingly, it should be understood that this sectionshould be read in this light, and not necessarily as admissions of priorart.

Waste Heat Recovery Units (WHRU) and similar systems transfer energyfrom higher temperature vapor or gas streams to a heating medium (HM)fluid, typically a liquid. The HM is typically utilized as a heat sourcefor other heat transfer equipment (users or load) in a target or desiredprocess. The source of the high-temperature vapor stream may be a gasturbine, fired heater, steam generator, and other sources. Thehigh-temperature vapor may be a hot exhaust gas, for example, from oneor more of these sources. At the WHRU, the high-temperature vaportypically flows through ductwork or other enclosure housing coils or abundle of tubes. The HM flows through the coils or tubes and absorbsheat from the high-temperature vapor stream passing over the coils ortubes.

In normal operation, the HM fluid typically flows through the coils ortubes at a flow rate and relatively-short residence time that preventsexcessive elevated temperature of the HM thus may avoid significantthermal degradation of the HM or excessive deposition of HM impurities,and the like. Unfortunately, during an abnormal operation or shutdowncondition, the HM flow may slow or stop. Thus, the HM fluid retained inthe coils or tubes may experience elevated temperatures and thermallydegrade. Indeed, at elevated temperature over time, glycols or othersimilar components of the HM may form sludges having corrosive acids.Further, certain HM fluids and components, e.g., water, may exceedtypical deposit rates of impurities on the tube wall at elevatedtemperature over time, fouling the tubes.

United States Patent Application Publication No. 2011/0067742 entitled“Thermoelectric-Based Power Generation Systems and Methods” to Bell etal. describes a waste heat recovery system with a cylindrical shell thatcontains an exhaust fluid. Two heat exchangers extend into thecylindrical shell to exchange heat with the exhaust fluid. A controlvalve determines when exhaust fluid may pass through the second heatexchanger.

United States Patent Application Publication No. 2011/0167865 entitled“Air-Conditioning Apparatus” to Morimoto et al. describes anair-conditioning apparatus in which the refrigerant exchanges heat witha HM in at least one intermediate heat exchanger. The document alsodescribes the circulation systems of both the refrigerant and the HM.

U.S. Pat. No. 4,669,530 entitled “Heat Exchanger Method and Apparatus”to Warner describes a dual-heat exchanger heat removal system forcooling sulfur trioxide. The design protects the first heat exchangerfrom corrosion by not permitting the sulfur trioxide to condense withinits assembly. Furthermore, the design protects the second heat exchangerfrom thermal damage by cooling the sulfur trioxide in the first heatexchanger.

U.S. Pat. No. 4,737,531 entitled “Waste Heat Recovery” to Rogersdescribes a heat exchange unit that recovers waste heat from carbonblack smoke by preheating oxygen-containing gas such as air passed to acarbon black producing reactor. A control loop is provided thatautomatically diverts the oxygen-containing gas being preheated througha by-pass line if the temperature of the effluent smoke removed from theheat exchange is below a minimum temperature value to minimize thedeposition of carbon on the heat exchange surfaces in the heat exchangezone.

U.S. Pat. No. 6,984,292 entitled “Water Treatment for Thermal Heavy Oil”to Kresnyak et al. includes initial steps of capturing the waste heatenergy from high-pressure steam separator located downstream of steamgenerators. It further describes operating conditions that promote a 1%to about 50% mass vapor in the stream returning to the heated separatorto prevent fouling and scaling.

U.S. Pat. No. 7,823,628 entitled “Passive Back-Flushing Thermal EnergySystem” to Harrison describes a thermal energy system having a heatexchanger for transferring thermal energy between a source and a load.The heat exchanger has a primary side associated with the source and asecondary side for conducting a fluid associated with the load. Thesecondary side of the heat exchanger is back-flushed via aspecially-designed back-flush valve upon a consumption of a portion ofthe fluid. The abstract explains that passive back flushing preventsfouling of the heat exchanger due to sediments, scale, and mineraldeposits that may be present in the circulating fluid.

U.S. Pat. No. 8,470,097 entitled “Silicon-Containing Steel Compositionwith Improved Heat Exchanger Corrosion and Fouling Resistance” to Chunet al. describes a method of providing sulfidation corrosion resistanceand corrosion induced fouling resistance to a heat transfer componentsurface.

A problem in a heat recovery system including a WHRU may be excessivetemperature and associated thermal degradation of the HM, such asglycol, in the waste heat recovery system.

SUMMARY

An embodiment relates to a heat recovery system having a tube bundlewith multiple tubes. The heat recovery system include an enclosurehousing at least a portion of the tube bundle, wherein the enclosure isconfigured to receive a vapor and pass the vapor over the tube bundle. Apump configured to flow a heating medium through the tube bundle to oneor more users. A control system is configured to detect an abnormaloperation of the heat recovery system, wherein the abnormal operationincludes a reduction of flow of the heating medium below a predeterminedrange of flow values. Lastly, the heat recovery system includes acollection system configured to reduce thermal degradation of theheating medium during the abnormal operation by evacuating the heatingmedium from the tube bundle during the abnormal operation.

Another embodiment relates to a waste heat recovery unit having a heatexchanger and a ductwork enclosing at least a portion of the coils,wherein the ductwork is configured to receive and pass an exhaust gasover the heat exchanger. The waste heat recovery unit includes a pumpconfigured to circulate a heating medium through an internal flow pathof the heat exchanger to absorb heat from the exhaust gas, and whereinthe pump is configured to circulate the heating medium from the wasteheat recovery unit to users of the heating medium. Further, the wasteheat recovery unit includes a collection system configured to remove theheating medium from the heat exchanger during an abnormal operation toreduce thermal degradation of the heating medium in the heat exchanger.The collection system includes a collection vessel configured to receiveheating medium removed from the heat exchanger during the abnormaloperation, and wherein the abnormal operation includes a cessation offlow or substantial reduction in a flow rate of the heating mediumthrough the heat exchanger.

Yet another embodiment relates to a method of operating a waste heatrecovery unit, the method including circulating a heating medium throughmultiple tubes of a tube bundle in the waste heat recovery unit and tousers of the heating medium outside of the waste heat recovery unit. Theexemplary method includes passing a vapor over the tube bundle totransfer heat from the vapor to the heating medium. Further, the methodincludes detecting, e.g., via a control system, an abnormal operation ofthe waste heat recovery unit, the abnormal operation including loss ofcirculation of the heating medium through the multiple tubes. Lastly,the method includes removing the heating medium from the multiple tubesto a collection vessel in the waste heat recovery unit during theabnormal operation to reduce or avoid thermal degradation of the heatingmedium.

Yet another embodiment relates to a method of constructing orretrofitting a waste heat recovery unit (WHRU) to prevent or reducethermal degradation of heating medium during abnormal operation of theWHRU, the method including adding a conduit downstream of a tube bundleconfigured to transfer heat from an exhaust gas to the heating medium,wherein the conduit is configured to divert flow of the heating mediumin the WHRU during the abnormal operation of the WHRU, theoff-specification operation including cessation of normal flow ofheating medium through the tube bundle. The exemplary method includesinstalling a collection vessel coupled to the conduit and configured toreceive heating medium drained from the tube bundle via the conduitduring the abnormal operation. Lastly, the method includes installing avalve on the conduit, wherein the valve is configured to isolate thecollection vessel during normal operation of the WHRU and to place thecollection vessel in service during the abnormal operation.

DESCRIPTION OF THE DRAWINGS

The advantages of the present techniques are better understood byreferring to the following detailed description and the attacheddrawings, in which:

FIG. 1 is a block flow diagram of an operational system having a wasteheat recovery unit (WHRU) and users of the HM circulating from the WHRU;

FIG. 2 is a block flow diagram of an exemplary WHRU of FIG. 1 having anHM collection system;

FIG. 3 is a schematic diagram of the exemplary WHRU of FIG. 2 having atube bundle coupled to the HM collection system;

FIG. 4 is a simplified process flow diagram of the exemplary WHRU ofFIG. 3 having an exemplary HM collection system;

FIG. 5 is a block diagram of an exemplary method of evacuating HM from aWHRU tube bundle during abnormal operation; and

FIG. 6 is a block diagram of retrofitting a WHRU to incorporate anexemplary HM collection system for abnormal operation.

DETAILED DESCRIPTION

In the following detailed description section, specific embodiments ofthe present techniques are described. However, to the extent that thefollowing description is specific to a particular embodiment or aparticular use of the present techniques, this is intended to be forexemplary purposes only and simply provides a description of theexemplary embodiments. Accordingly, the techniques are not limited tothe specific embodiments described below, but rather, include allalternatives, modifications, and equivalents falling within the truespirit and scope of the appended claims.

As used herein, “substantially”, “predominately” and other words ofdegree are relative modifiers intended to indicate permissible variationfrom the characteristic so modified. It is not intended to be limited tothe absolute value or characteristic which it modifies, but ratherpossessing more of the physical or functional characteristic than itsopposite, and preferably, approaching or approximating such a physicalor functional characteristic.

“Exemplary” is used exclusively herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not to be construed as preferred or advantageous overother embodiments.

The term “gas” is used interchangeably with “vapor,” and means asubstance or mixture of substances in the gaseous state as distinguishedfrom the liquid or solid state. Likewise, the term “liquid” means asubstance or mixture of substances in the liquid state as distinguishedfrom the gas or solid state. As used herein, “fluid” is a generic termthat may include liquid and either a gas or vapor.

Embodiments of the present techniques are directed a thermal energysystem such as a heat recovery system, waste heat recovery unit (WHRU),or similar system. The thermal energy system has a heat exchanger fortransferring thermal energy between thermal energy content and a load.The thermal energy content may include a high temperature vapor, such asan exhaust gas, while the load may include a heat transfer fluid, a HM,and the like. The high-temperature vapor may be provided or produced bygas turbines, fired heaters, steam generators, and so forth.

In examples, the heat exchanger has a primary side associated with thehigh-temperature vapor, and a secondary side for receiving andoutputting the load, e.g., the secondary side receives a fluid to beheated and outputs a heated fluid. The heat exchanger may reside, forinstance, in a ductwork or other enclosure transporting the energysource fluid. As discussed below, auxiliary equipment and a controlsystem may provide for shutdown conditions and evacuation of thesecondary side, e.g., tube-side, of the heat exchanger.

A bundle or coils of tubes may form the assembly of the heat exchangertransferring energy between the high-temperature vapor and load fluids.The tubes of the bundle, or coil of tubes, of the heat exchanger may beplain, externally finned, internally finned, continuously finned and maypossess numerous other heat-transfer enhancements or features. Theconduits or tubes of the bundle/coil of tubes may have a downward slopeto aid drainage of HM fluid. The upward movement or escape of formed HMfluid vapor may also be assisted. The HM will generally exchange energywith the high-temperature vapor while remaining in the liquid phase. Thesystem may include tanks, valves, and associated piping to evacuate theHM fluid from the heat exchanger in the case of failed circulation ofthe HM fluid.

Further, air or ambient air may be introduced during the shutdown periodto reduce the temperature of the high-temperature vapor flowing acrossthe primary side, e.g., the exterior, of the heat exchanger. The reducedtemperature from the introduction of air may decrease the thermaldegradation of the HM fluid on the secondary side of the heat exchanger.The system may also introduce an inert gas purge through the tube sideof the heat exchanger to remove any residual heating-medium film on theinner surface of the secondary side of the heat exchanger. This may alsomitigate the formation of a solid and corrosive fouling layer on eitherside of the heat exchanger, due to oxidation.

In the WHRU's transfer of energy from higher temperature vapor streamsto a HM fluid, typically a liquid, the HM may be utilized as a heatsource for other heat transfer equipment, such as users or other loads,in a desired or target process. As discussed, in the WHRU, thehigh-temperature vapor that is carried in the ductwork typically passesover a bundle of tubes or coils through which the HM fluid flows.Various physical states and compositions of the HM fluid may be employedbut the HM fluids may generally thermally degrade or deposit impuritiesor scale when maintained at excessive elevated temperatures over time.Thermal degradation of glycols, for example, a component of certain HMfluids, may form sludges of corrosive acids. With continued exposure tothe effects of the high-temperature vapor on the outer tube wall, theformed corrosive sludge may continue to degrade and transform into asubstantially solid layer of material on the inner tube wall. The solidmaterial may negatively affect the mechanical integrity and operationalperformance of WHRU tube bundle and associated process equipment orusers of the HM.

In normal operation, the HM fluid flows inside the tubes at a rate thatmay substantially avoid thermal degradation of the HM fluid, includingat the inner tube wall that typically exhibits the highest temperaturein the tubes. However, during off-specification or abnormal operatingconditions, the HM fluid flow inside the tubes may slow or even stop. Ifthe tube bundle remains full of stagnant HM fluid exposed to thehigh-temperature vapor flow, thermal degradation of the HM fluid insidethe tubes may begin, for example, starting at the inner tube wall.Again, the thermal degradation may lead to the formation of corrosivesludge that negatively impacts and reduces the mechanical integrity ofthe tubes, and may lead to formation of an associated fouling layer thatgenerally worsens the performance of the WHRU. Off-specification orabnormal operating conditions may result in a reduction of or stopped HMfluid circulation flow, higher vapor temperatures than design on theoutside of the tubes, and failure of a control damper within the WHRUassembly to divert the high-temperature vapor flow away from the tubebundle when desired, and so forth. In the event that the HM circulationdecreases below an acceptable range and the high-temperature vapor isnot diverted away from the WHRU coils either by design or systemfailure, the described apparatus and generalized strategy of itsoperation is expected to protect the HM from thermal degradation and,therefore, protect its associated equipment.

FIG. 1 is a block flow diagram of an operational system 100 having aheat recovery unit such as a waste heat recovery unit (WHRU) 102. Ingeneral, a WHRU recovers heat from hot exhaust vapor in the system 100before discharging the hot exhaust vapor. In the recovery, the WHRUtransfers heat from the incoming hot exhaust vapor to a heat-transferfluid. Thus, the recovered heat is used to increase the temperature ofthe heat-transfer fluid. The heated heat-transfer fluid may be suppliedas a utility HM to process equipment in the system 100.

In operation in the illustrated embodiment, the HM circulates throughthe WHRU 102 to absorb heat from hot exhaust vapor and then through oneor more users 104 in the system 100 to provide heat to the users 104.The users 104 may be heat exchangers or other types of equipment thatabsorb heat from the HM. The overall system 100 may be an industrial,commercial, or public facility or complex involved in the manufacturing,production, processing, treatment, handling, and so forth, of one ormore products (not shown).

In embodiments, the WHRU 102 circulates the HM through a heat exchangerto absorb heat from an incoming hot exhaust vapor to provide a HM supply106 to the users 104. The WHRU 102 receives a HM return 108 from theusers 104. The HM return 108 is typically cooler than the HM supply 106.Again, the users 104 may be heat exchangers or other process equipmentthat absorb heat from the HM supply 106.

In some embodiments, the WHRU 102 receives a vapor 110 from one or moresources 112, and discharges a cooled vapor 114. The cooled vapor 114 maybe discharged to atmosphere or to a recovery system, and so on. Theinlet vapor 110 is generally a high-temperature vapor, such as a hotexhaust gas, and the like. The sources 112 of the inlet vapor 110 may begas turbines, fired heaters, steam generators, and other sources of hotvapor or gas. In examples, the inlet vapor 110 may be waste gas from thesource 112. An exemplary temperature range of the inlet vapor 110 isabout 300° C. to about 800° C. An exemplary temperature range of thecooled vapor 114 discharging from the WHRU 102 is about 200° C. to about550° C. Of course, other temperature range values for the inlet vapor110 and the cooled vapor 114 are applicable. In certain embodiments, theinlet vapor 110 is a discharge or exhaust gas or vapor from one or moresources 112 outside of the WHRU 102. In other words, the WHRU 102 may bea utility system that receives inlet vapor 110 from process orproduction systems or other utility systems. In fact, the WHRU 102 maybe a shared utility in the overall system 100 at a facility or site, forexample. As appreciated by one of ordinary skill in the art, the WHRU102 may be a utility system outside the battery limits of the source(s)112. Indeed, in examples, the WHRU 102 may be an outside battery limits(OSBL) system to the source(s) 112.

Further, the overall system 100 and/or WHRU 102 may include a controlsystem 116, such as a distributed control system (DCS), programmablelogic controller (PLC), and so on. The control system 116 may have ahuman interface (HMI) and facilitate control of the system 100 includingthe WHRU 102. The control system 116 may direct operation of equipment,control valves, and the like, in the WHRU 102. The control system 116may include instrumentation, computers, computer memory, a processor,and so forth. The control system 116 may include logic or code stored inmemory and executable by the processor to implement or facilitate thecontrol actions disclosed herein. Lastly, as discussed below, the WHRU102 may include a shutdown HM collection system (that may work inconcert with the control system 116) to reduce thermal degradation ofthe HM during abnormal operations of the WHRU 102.

FIG. 2 is a block flow diagram of an exemplary WHRU 102 of FIG. 1 havingan exhaust stack 200 and a HM collection system 202. Like numbered itemsare as described with respect to FIG. 1. The exhaust stack 200 may housea heat exchanger (not shown) having coils or a tube bundle to transferheat from the inlet vapor 110 to a HM, e.g., to provide a heated HMsupply 106. The HM collection system 202 is coupled to the tube bundlein the stack 200 via a first or supply conduit 204 and a second orreturn conduit 206. As discussed below, the supply conduit 204 andreturn conduit 206 may accommodate bi-directional flow during abnormaloperation.

The HM collection system 202 may both facilitate circulation of the HMincluding during normal operation, and facilitate the evacuation andcollection of HM from the heat-exchanger tubes or coils in the stack 200during a shutdown or abnormal operation. The HM collection system 202may include the pump and hydraulic expansion vessel that circulates theHM. Thus, the HM collection system 202 may provide the HM supply 106 tousers 104 (FIG. 1) and receives the HM return 108 from the users 104. Inthe illustrated embodiment during normal operation, the HM supply 106flows through the supply conduit 204 from the tube bundle or coils inthe stack 200, and the HM return 108 flows through the return conduit206 to the tube bundle of coils in the stack 200.

During abnormal operation with the HM circulation slowing or stopping,and with the high-temperature vapor 110 continuing to flow across theheat-exchanger tubes or coils in the stack 200, the HM collection system202 may facilitate evacuation of the HM from the tube coils in the stack200. The evacuation of the HM from the tube coils may reduce thermaldegradation of the HM during the shutdown or abnormal operation.

In some configurations, the HM collection system 202 includes acollection vessel 208 to accumulate HM drained via the supply conduit204 from the coils during the abnormal or off-specification operation.The collection vessel may be sized to at least the volume of theheat-exchanger tubes or coils in the stack 200. Also, in certainexamples, any vaporized HM from the coils during the off-specificationoperation may flow through the return conduit 206 to the HM collectionsystem 202. If desired, the HM vapor may further discharge through avent 210 from the collection system 202.

Additionally, the HM collection system 202 may flow an inert gas 212purge through the coils or tubes in the stack 200 via the supply conduit204, for example, to facilitate removal of residual HM film from theinner surface of the coils or tubes. The purge may reduce the formationof a solid and corrosive fouling layer in the tubes. Moreover, a gas 214such as ambient air may be introduced into the stack 200 during theabnormal operation or off-specification operation to reduce thetemperature of inlet vapor 110 flowing across the tubes or coils in thestack 200. Thus, the introduction of the gas 214 may contribute to thereduction of the thermal degradation of the HM fluid in the tubes orcoils. Lastly, the WHRU 102 may employ the control system 116 tofacilitate implementation of the HM evacuation from the tubes and theassociated aforementioned operations. For example, abnormal operationmay include a shutdown or a decrease in flow that may cause the HM fluidto reach temperatures that may result in degradation. Accordingly, ameasurement of the temperature of the HM may be used to determinewhether the HM needs to be drained from the coils, alone or incombination with other indications, such as a failure of a circulationpump.

FIG. 3 is a schematic diagram of the exemplary WHRU 102 of FIGS. 1 and 2having a heat exchanger as a tube bundle 300 in the stack 200. Likenumbered items are as described with respect to FIGS. 1 and 2. The tubesor coil of tubes of the bundle 300 of the heat exchanger may be plain,externally finned, internally finned, continuously finned and maypossesses other heat-transfer enhancements or features. The WHRU 102 mayhave one or more heat exchangers, each having respective tube bundles300, disposed in series, parallel, or both.

In the illustrated embodiment, the tube bundle 300 is disposed in aprimary stack 302 and coupled to the HM collection system 202. Thecoupling provides for operational or fluidic coupling of the HMcollection system 202 to the tube-side of the tube bundle 300. The stack200 includes the primary stack 302 and a bypass stack 304. The primarystack 302 and the bypass stack 304 may be ductwork and/or other types ofenclosures and conduits. If ductwork is employed, the ductwork may be aseries of ducts. As appreciated by the skilled artisan in this context,a duct may be an enclosure, conduit, tube, or pipe for conveying thevapor 110. The material of construction of the ductwork may be selectedfor relatively high temperature to accommodate the temperature of thevapor 110. In certain embodiments with the primary stack 302, theductwork or portions of the ductwork conveys vapor 110 or other fluidsacross the tube bundle 300, and also houses or encloses, or partiallyencloses, the tube bundle 300.

The primary stack 302 is generally employed during typical operation,whereas the bypass stack 304 may generally be placed in operation duringatypical operation. To activate the bypass stack 304 in the illustratedembodiment, a damper 306 may be opened to divert vapor 110 flow from theprimary stack 302 to the bypass stack 304. The diverted vapor 110 maydischarge from the bypass stack 304, as indicated by arrow 308, to theatmosphere or for additional processing. In embodiments, the bypassstack 304 may include various operational units 310, such as a silenceror exhaust treatment catalyst bed.

As mentioned, the primary stack 302 houses the tube bundle 300. Theinlet vapor 110 enters the primary stack 302 and passes across the tubebundle 300. The WHRU 102 employs the tube bundle 300 to transfer heatfrom the vapor 110 to the HM flowing through the tubes of the tubebundle 300. A cooled vapor 114 flowing from across the tube bundle 300discharges from the primary stack 302 to the atmosphere or additionalprocessing.

The HM having absorbed heat from the vapor 110, and thus, a highertemperature, discharges from the tube-side of the tube bundle 300 intothe supply conduit 204 and is provided as HM supply 106 to users 104(FIG. 1). The HM return 108 from the users 104 flows through the returnconduit 206 to the tube-side of the tube bundle 300 to be reheated inthe tube bundle 300.

During an abnormal operation, such as a pump failure or system shutdownthat involves the HM circulation slowing or discontinuing, the HMcollection system 202 may facilitate evacuation of HM from the tube-sideof the tube bundle 300. Such evacuation may reduce thermal degradationof the HM during the shutdown or abnormal operation, especially whilethe high-temperature vapor 110 is continuing to flow across the tubebundle 300 during the abnormal operation. The continued flow of thehigh-temperature vapor 110 across the tube bundle 300 may be intended orby design, or may be due to mechanical failure or faulty operation ofthe damper 306, problems with the bypass stack 304, and so forth.

As discussed, the HM collection system 202 may include a collectionvessel 208 to accumulate HM drained via the supply conduit 204 from thetube bundle 300 during the abnormal or off-specification operation. Thetube bundle 300 and the supply conduit 204 may be configured (e.g., amild slope) to facilitate flow or draining, e.g., via gravity, of the HMfrom the tube bundle 300 to the collection vessel 208.

In some embodiments, a back-flushing of the tube bundle 300 to displaceor remove HM from the tube bundle 300 is not required. Indeed, certainembodiments of the HM collection system 202 are configured whereinback-flushing is not employed to remove HM from the tube bundle 300during the abnormal operation.

Moreover, any vaporized HM from the tube bundle 300 during the shutdownor off-specification operation may flow from the tube bundle through thereturn conduit 206 to the HM collection system 202 and discharge as avent 210. This HM vapor flow (if present) is generally in the oppositedirection of the typical HM return 108 liquid flow through the returnconduit 206 during normal operation.

As also mentioned, the HM collection system 202 may include piping tointroduce an inert gas 212 into the tube-side of the tube bundle 300 viathe supply conduit 204 or other conduit. The purging of the tubes withthe inert gas 212 may facilitate removal of residual HM film from theinner surface of the coils or tubes, reduce formation of a solid andcorrosive fouling layer in the tubes, and the like. In examples, theinert gas 212 may flow through supply conduit 204 (in opposite directionof normal flow of the HM supply 106), through the tubes of the tubebundle 300, and then through the return conduit 206 (in oppositedirection of normal flow of the HM return 108) to be discharged from theHM collection system 202 in the vent 210.

FIG. 4 is a simplified process flow diagram of the exemplary WHRU 102 ofFIGS. 1-3 having an exemplary HM collection system 202. In this example,the HM collection system 202 is coupled to the tube bundle 300 via thesupply conduit 204 and return conduit 206. In the illustratedembodiment, the HM collection system 202 drives both circulation of HMduring normal operation and the collection of HM from the tube bundle300 (e.g., to a HM collection vessel 208) during certain abnormaloperations of the WHRU 102.

The HM collection system 202 includes a HM expansion vessel 402 and apump 404 to circulate the HM including during normal operation. The pump404 provides a motive force to circulate the HM through the HM circuitincluding through the tube bundle 300 and the users 104. In examples,the pump 404 is a centrifugal pump. The HM expansion vessel 402accommodates thermal expansion of the HM flowing through thehydraulically-full HM circuit. In particular, the liquid level 406 inthe expansion vessel 402 may rise and fall to account for thermalexpansion of the HM in the circuit.

A vent valve 408, such as a control valve, pressure control valve, orflow control valve, among others, which is located on the expansionvessel 402 overhead may be modulated to prevent excessive accumulationof uncondensed components in the system and to control pressure of theexpansion vessel 402 to a desired pressure set point. The valve 408 mayvent noncondensable or uncondensed components, as vent 210, thataccumulate in the circuit. In operation, the vent valve 408 may modulatethe flow rate of a vent 210 stream from the vapor space of the expansionvessel 402 to control pressure in the expansion vessel 402, and thus,facilitate control of suction pressure of the HM circulating pump 404.

In the illustrated embodiment, the expansion vessel 402 receives the HMreturn 108 from the users 104 (FIG. 1). The HM return 108 flows from theexpansion vessel 402 via pump suction piping 410 to the pump 404. Thepump 404 discharges the HM return 108 through pump discharge piping 412and return conduit 206 to the tube-side of the tube bundle 300 in theprimary stack 302. Isolation valves may be disposed around thecirculation pump 404 on the suction piping 410 and the discharge piping412, respectively. In the illustrated example, an isolation block valve416 is disposed on the pump discharge piping 412. The valve 416 mayadditionally be a modulating control valve to adjust flow rate of theHM. On other hand, in the case of the valve 416 as only an isolationvalve, an additional valve (not shown) as a control valve may bedisposed on the pump discharge piping 412 or the return conduit 206 tomodulate the flow rate of the HM. Any such control valves, as well aspump recirculation line 418 and recirculation valve 420, may promotehydraulic stability of the pump 404 operation. Typically, the pump 404provides motive force in the HM circulation for transport of the HMsupply 106 from the WHRU 102 to the users 104. In operation, the HMsupply 106 flows from the tube bundle 300 through the supply conduit204, supply piping 422, and any supply valves 424 and 426 on the supplypiping 422, to the users 104.

As discussed, removal of the HM fluid from the tube-side of the tubebundle 300 may be implemented during certain abnormal operations of theWHRU 102 to avoid or reduce thermal degradation of the HM. To implementsuch removal, an HM collection vessel 208 disposed on the downstreamside of the tube bundle 300 may be placed in service, such as by openingvalves 428 and 430, e.g., block valves, around the collection vessel208. Of course, other valve and piping configurations to place thecollection vessel 208 in operation may be applicable. Moreover, in someembodiments, one or more optional valves 424 and 426 on the piping 422that transports the HM supply 106 to users during normal operation maybe closed in the shutdown or off-specification scenario.

Typically, during normal operation, the HM collection vessel 208 isisolated, e.g., with valves 428 and 430 in a closed position, and is notutilized during normal operation of the WHRU 102. Again, the HM supply106 during normal operation flows through supply piping 422 and with anysupply valves 424 and 426 in an open position. However, as mentioned,during an abnormal operation of the WHRU 102, the collection vessel 208and collection piping 432 placed in service to receive HM fluid from thetube bundle 300 to the collection vessel 208 via the supply conduit 204and collection piping 432.

Indeed, in abnormal operation where the circulating HM fluid slows orstops, such as due to a pump 404 failure, and which may include exampleswith the high-temperature inlet vapor 110 continuing to flow across thetube bundle 300, the HM fluid may be evacuated from the tube bundle 300to the collection vessel 208. Otherwise, the stagnant HM fluid in thetube bundle 300 may reach elevated temperatures and thermally degrade.Therefore, in such abnormal conditions, the HM fluid may be removed,e.g., drained, to the collection vessel 208 via collection piping 432.

As indicated, to place the collection vessel 208 in service, at leastone block valve around the collection vessel 208 may be opened. In theillustrated embodiment, to place the collection vessel 208 in operation,an inlet valve 428 to the collection vessel 208 and an outlet valve 430from the collection vessel 208 may each be opened. Thus, a collectionconduit or collection piping 432 and the collection vessel 208 areplaced in service for the abnormal operation and to remove HM from thetube bundle 300. Further, in addition to the collection vessel 208 andcollection piping 432 placed in operation to receive HM fluid evacuatedor drained from the tube bundle 300, one or more valves 424 and 426 (ifpresent) on the HM supply 106 piping 422 to the users 104 may beoptionally placed in a closed valve position.

Further, in embodiments, HM fluid vaporized (if present) in the tubebundle 300 may be removed. For example, such HM vapor may flow throughreturn conduit 206 and recirculation line 418 to the expansion vessel402, and discharged by the vent valve 408. Moreover, it should be notedthat if a pump recirculation line 418 is not included in the WHRU 102, asimilar line 418 and valve 420 may be installed to provide a route forHM vapor to reach the vapor space of the expansion vessel 402 from thetubes or coils during the abnormal operation. In addition, an inert gas212 may be purged through the tube bundle 300. The inert gas 212 mayreach the tube bundle 300 through the supply conduit 204, for example.Of course, other various pipes or conduits may be utilized to introducethe inert gas 212 purge through the tube bundle 300.

Thus, in an event that the HM circulation rate decreases below anacceptable value, the aforementioned techniques may protect the HM fluidfrom substantial thermal degradation. This may be especially useful ifthe high-temperature vapor, e.g., hot exhaust gas, is not diverted fromthe WHRU tubes or coils. This shutdown technique may reduce fouling ofthe WHRU coil or tubes, and also users of the HM from the WHRU.

A control system may be configured to detect an abnormal operation of aheat recovery system or the WHRU, wherein the abnormal operation mayinclude a reduction of flow of the HM below a predetermined range offlow values. An inadvertent or intentional shutdown of the circulationpump 404 may be the cause of the reduced or no circulation flow in theabnormal operation. An inadvertent shutdown of the pump 404 may involvean operational mistake and/or equipment failure. Again, the pump 404 maybe a pump configured to flow a HM through the tube bundle 300 to one ormore users.

In one example of abnormal operation, the HM circulation pump 404 may beelectrically tripped and thus stopped. In other words, the electricalsupply to the motor of the pump 404 is stopped and thus the pump 404does not provide a motive force for flow of the HM through the tubebundle 300. This represents a viable cause of reduced HM circulationrate and/or cessation of HM circulation. Of course, other causes mayresult in a reduced or ceased HM circulation.

To implement a shutdown and HM evacuation technique to reduce thermaldegradation of the HM in the tube bundle 300 or coils, the pump 404 maybe isolated from the HM fluid in the tube bundle 300. Further, therecirculation valve 420 may be placed in an open or full open positionto facilitate any potentially expanding HM liquid or vapor in the tubebundle 300 to travel in a reverse flow direction through the returnconduit 206. HM vapor generated in the tube bundle 300 during theabnormal operation and shutdown may pass through the recirculation valve420 and out through the vent valve 408 as needed. Optionally, a blockvalve 434 may be employed to the users from the HM expansion vessel 402during the shutdown period.

With the flow of the primary HM fluid circuit stopped, and potentiallyblocked via the valve 434 at the expansion vessel 402, the valve 428 tothe HM collection vessel 208 may be opened to facilitate HM fluidinventory within the tube bundle 300 to drain freely (by gravity) intothe HM collection vessel 208. Opening this side stream to the collectedvessel 208 may reduce the amount of time the HM fluid is subject to theharsh process conditions in the tube bundle 300, such as withcontinuation of the high-temperature vapor 110 flowing over tube bundle300 while the pump 404 is shut down and with no normal flow of HMthrough the tube bundle 300. This opening of the valve 428 to thecollection vessel 208 may reduce the vaporization of the HM fluid, thedegradation of the HM fluid, and the formation of corrosive sludge inthe non-flowing HM fluid, and so on. The tubes of tube bundle 300 maypossess a downward slope to aid downward HM fluid liquid drainage to theHM collection vessel 208 and facilitate upward escape of formed HM fluidvapor escape to the HM expansion vessel 402 in this shutdownarrangement.

A gas 212 such as ambient air may be introduced into the stack 302during the shutdown period to dilute and lower temperature of thehigh-temperature vapor 110, e.g., hot exhaust gas, flowing over the tubebundle 300. Such ambient air introduction may aid the mitigation of thethermal degradation of the HM fluid within the tube bundle 300 duringthe off-specification operation and shutdown period. Furthermore, theaforementioned inert gas 212 purge may pass through the tubes of thetube bundle to assist in purging any film of HM fluid that remains onthe inside walls of the tubes.

Lastly, these exemplary configurations specifically related to exemplaryvalve and equipment arrangements are not meant to limit the presenttechniques. Other arrangements within the scope of the presenttechniques may implemented to circulate the HM, and also to reduce HMfluid thermal degradation and associated damage to equipment in responseto various abnormal operation and circuit shutdown causes.

For example, the pump 404 may be located on the HM return portion of thecircuit before the inlet to the coils or tube bundle 300, as shown, ormay instead be disposed on the HM supply portion of the circuit afterthe outlet of the tube bundle 300. Indeed, in some embodiments, the pump404 may generally be disposed at locations throughout the HM system andcircuit. Further, in certain examples, the pump 404 may not be acomponent of the collection system 202 but instead be disposed outsideof the collection system 202. Furthermore, in some embodiments, thecollection vessel 208 may be generally in addition to the expansionvessel 402, as shown, wherein the collection vessel is not the expansionvessel.

Moreover, the HM flow through the coils or tube bundle 300 may bedownward or upward with respect to ground. Also, the enclosure orprimary stack 302 routing the inlet vapor 110 may be vertical,horizontal, or inclined, or any combination thereof.

FIG. 5 is a block diagram of a method 500 of operating a heat transfersystem such as a waste heat recovery unit. At block 502, the methodincludes circulating a HM through multiple tubes or coils of a tubebundle in a waste heat recovery unit and to users of the HM. At block504, the method includes passing a vapor over the tube bundle totransfer heat from the vapor to the HM. The vapor may be an exhaust gasfrom an upstream source.

At block 506, the method includes detecting, e.g., via a control system,an abnormal operation of the waste heat recovery unit. The abnormaloperation may include loss of normal circulation of the HM through themultiple tubes or coils, an excessive temperature of the HM in orexiting the tube bundle, and so on. For instance, the HM flow may slowor stop through the tubes. In examples, the loss of circulation of theHM is caused by the failure or shutdown of a pump that circulates theHM. Further, the abnormal operation may include continued flow of thevapor over the tube bundle or coils. In some circumstances, the reducedflow may be caused by an operational error from a user 104 (FIG. 1), forexample, closing a valve in a separate process unit.

At block 508, the method includes evacuating the HM from the multipletubes or coils to a collection vessel during the abnormal operation toreduce thermal degradation of the HM. The waste heat recovery unit has aHM collection system having the collection vessel. Evacuating of the HMmay include draining by gravity the HM from the multiple tubes to thecollection vessel. The evacuation of the HM may substantiallydeinventory or evacuate the HM from the multiple tubes or coils.Moreover, the evacuating of the HM from the tubes or coils may includeventing HM vaporized in the multiple tubes or coils during the abnormaloperation.

At block 510, the method may include diluting the vapor with air toreduce temperature of the vapor during the abnormal operation. The airmay be ambient air introduced into the enclosure or ductwork receivingthe vapor and housing the tube bundle. At block 512, the method mayinclude purging the multiple tubes or coils with an inert gas during theabnormal operation.

FIG. 6 is a block diagram of a method 600 of constructing orretrofitting a waste heat recovery unit (WHRU) to reduce thermaldegradation of HM during abnormal or off-specification operation of theWHRU. In particular, the method 600 may involve construction of a newWHRU or retrofit of an existing WHRU to provide for deinventory orevacuation of HM from tubes or coils in the WHRU duringoff-specification or abnormal operation to reduce thermal degradation ofthe HM. For example, the new construction or retrofit may allow HM inthe tubes or coils to be drained during the abnormal operation. At block602, the method includes adding a conduit downstream of a tube bundle inthe WHRU that transfers heat from an exhaust gas to the HM, wherein theconduit to divert flow of the HM in the WHRU during the abnormaloperation of the WHRU. The abnormal operation may include cessation ofnormal flow of HM through tubes or coils of the tube bundle, such aswith shutdown or failure of a HM circulation pump, closure of a valve onthe HM circuit, and so forth. At block 602, the method includesinstalling a collection vessel coupled to the conduit and to receive HMdrained from the tube bundle via the conduit during the abnormaloperation. The collection vessel may be sized to hold at least theamount of HM in the tubes or coils of the tube bundle during normaloperation. Moreover, the collection vessel and associated piping andvalves, may be carbon steel, stainless steel, nickel alloys, exoticmetals, fiberglass reinforced plastic (FRP), and so forth. At block 604,the method includes incorporating a valve on the conduit to facilitateisolating the collection vessel during normal operation of the WHRU andto facilitate placing the collection vessel in service during theabnormal operation. For example, the valve (see, e.g., valve 428 in FIG.4) in a closed position may isolate, at least in part, the collectionvessel during normal operation of the WHRU, and the valve in an openposition may facilitate the collection vessel to be placed in serviceduring the abnormal operation of the WHRU. At block 606, the methodincludes installing an inert-gas purge inlet to provide an inert gaspurge (e.g., nitrogen) through tubes or coils of the tube bundle duringthe off-specification operation. At block 608, the method includesinstalling another conduit to receive vaporized HM from the tube bundleduring off-specification operation. At block 610, the method includesinstalling an ambient-air inlet on ductwork receiving the exhaust gasand housing the tube bundle. Lastly, at block 612, the method mayinclude integrating or programming a control system (e.g., DCS, PLC,etc.) to detect the abnormal or off-specification operation, and toautomatically implement operation of the aforementioned features. To doso, the control system may have a processor and code or logic stored onmemory and executable by the processor. The control system viainstrumentation may detect, for example, a HM circulation flow ratefalling below a predetermined value, an “off” operation of the HMcirculation pump, a HM temperature above a predetermined value, and soforth. In response, the control may be configured (including programmed)to automatically adjust opening positions of valves to place thecollection in service to drain HM liquid from the tubes or coils.

In summary, Waste Heat Recovery Units (WHRU) and similar systemstransfer energy from higher temperature vapor or gas streams to a HMfluid, typically a liquid. The HM is generally utilized as a heat sourcefor other heat transfer equipment (users or load) in a target process.The source of the high-temperature vapor stream may be a gas turbine,fired heater, steam generator, and other sources. At the WHRU, thehigh-temperature vapor typically flows through ductwork or otherenclosure housing coils or a bundle of tubes. The HM flows through thecoils or tubes and absorbs heat from the high-temperature vapor streampassing over the coils or tubes.

In normal operation, the HM fluid may flow through the coils or tubes ata flow rate that provides a relatively-short residence time, whichprevents excessive elevated temperature of the HM, significant thermaldegradation of the HM, excessive deposition of HM impurities, and thelike. Unfortunately, during an abnormal operation or shutdown condition,the HM flow may slow or stop. Thus, the HM fluid in the coils or tubesmay experience elevated temperatures and thermally degrade. Indeed, atelevated temperature over time, glycols or other similar components ofthe HM may thermally degrade to corrosive acids and sludge. Further,certain HM fluids and components, e.g., water, at elevated temperaturesover time may exceed typical deposit rates of impurities on the tubewall, fouling the tubes.

In conclusion, embodiments herein may provide for a heat recovery systemwith a tube bundle having multiple tubes or coils. The system includesan enclosure (e.g., ductwork) housing at least a portion of the tubebundle, wherein the enclosure to receive and pass a vapor over the tubebundle. The vapor may be hot exhaust gas from an upstream source. Thesystem includes a pump to circulate a HM through the multiple tubes orcoils to absorb heat from the vapor, and the pump to circulate the HM tousers of the HM. A control system is employed to detect an abnormaloperation of the heat recovery unit, the abnormal operation including acessation or substantial reduction of a flow rate of normal flow of theHM through the multiple tubes. The abnormal operation may involve ashutdown of the pump. Further, the abnormal operation may includecontinued flow of the vapor across the tube bundle in the enclosure.

The heat recovery system includes a collection system to deinventory orevacuate the HM from the multiple tubes during the abnormal operation toreduce thermal degradation of the HM during the abnormal operation. Thecollection system may include a collection vessel to accumulate the HMremoved from the multiple tubes during the abnormal operation. Thecollection vessel may be sized to hold at least the amount of HM in themultiple tubes during normal operation. The collection system may alsoreceive and vent HM vaporized in the multiple tubes during the abnormaloperation. Additionally, the collection system may have an inert gasinlet to provide an inert gas purge through the multiple tubes duringthe abnormal operation. Also, the heat recovery system may have anambient air inlet on the enclosure to introduce ambient air into theenclosure to dilute and reduce temperature of the vapor during theabnormal operation.

Other embodiments provide for a WHRU having a heat exchanger (e.g.,coils, tube bundle, tubes, etc.) and ductwork enclosing at least aportion of the heat exchanger, wherein the ductwork to receive and passan exhaust gas over the heat exchanger. The WHRU includes a pump tocirculate a HM through an internal flow path of the heat exchanger toabsorb heat from the exhaust gas, and the pump to circulate the HM tousers of the HM. Further, the WHRU includes a collection system toremove the HM from the coils during an abnormal operation to reducethermal degradation of the HM in the heat exchanger. The collectionsystem has a collection vessel to receive HM removed from the heatexchanger during the abnormal operation. The collection system typicallyincludes a conduit to drain by gravity the HM from the heat exchanger tothe collection vessel. The abnormal operation may include a cessation inflow or a reduction in a flow rate of the HM through the heat exchanger.The abnormal operation may include or be caused by a shutdown of thepump, an accidental closure of a valve by a user, and the like.Moreover, during the abnormal operation, the exhaust gas may continue toflow across the coils. Lastly, the WHRU may include a control system todetect the abnormal operation, and to automatically isolate the wasteheat recovery unit from the users and to facilitate implementation ofremoval of the HM from the heat exchanger during the abnormal operation.

While the present techniques may be susceptible to various modificationsand alternative forms, the exemplary embodiments discussed above havebeen shown only by way of example. However, it should again beunderstood that the techniques is not intended to be limited to theparticular embodiments disclosed herein. Indeed, the present techniquesinclude all alternatives, modifications, and equivalents falling withinthe true spirit and scope of the appended claims.

What is claimed is:
 1. A heat recovery system comprising: a tube bundlehaving multiple tubes; an enclosure housing at least a portion of thetube bundle, wherein the enclosure is configured to receive a vapor andpass the vapor over the tube bundle; a pump configured to flow a heatingmedium through the tube bundle to one or more users; a control systemconfigured to detect an abnormal operation of the heat recovery system,wherein the abnormal operation comprises a reduction of flow of theheating medium below a predetermined range of flow values; and acollection system configured to reduce thermal degradation of theheating medium during the abnormal operation by evacuating the heatingmedium from the tube bundle during the abnormal operation.
 2. The heatrecovery system of claim 1, wherein the enclosure comprises ductwork. 3.The heat recovery system of claim 1, wherein the vapor comprises exhaustgas from a source outside of the heat recovery system.
 4. The heatrecovery system of claim 1, wherein the abnormal operation comprises ashutdown of the pump.
 5. The heat recovery system of claim 1, whereinthe abnormal operation comprises continued flow of the vapor across thetube bundle in the enclosure.
 6. The heat recovery system of claim 1,wherein the collection system comprises a collection vessel configuredto accumulate the heating medium removed from the multiple tubes duringthe abnormal operation.
 7. The heat recovery system of claim 1, whereinthe collection system is configured to evacuate the heating medium fromthe tube bundle without back-flushing the multiple tubes.
 8. The heatrecovery system of claim 1, wherein the collection system is configuredto receive and vent heating medium vaporized in the tube bundle duringthe abnormal operation.
 9. The heat recovery system of claim 1, whereinthe collection system comprises a purge gas inlet configured to passpurge gas through the tube bundle during the abnormal operation, whereinthe purge gas is inert.
 10. The heat recovery system of claim 1,comprising an ambient air inlet on the enclosure configured to introduceambient air into the enclosure to dilute and reduce temperature of thevapor.
 11. The heat recovery system of claim 1, wherein the tube bundlecomprises multiple tubes comprising coils.
 12. A waste heat recoveryunit comprising: a heat exchanger; a ductwork enclosing at least aportion of the coils, wherein the ductwork is configured to receive andpass an exhaust gas over the heat exchanger; a pump configured tocirculate a heating medium through an internal flow path of the heatexchanger to absorb heat from the exhaust gas, and wherein the pump isconfigured to circulate the heating medium from the waste heat recoveryunit to users of the heating medium; a collection system configured toremove the heating medium from the heat exchanger during an abnormaloperation to reduce thermal degradation of the heating medium in theheat exchanger, wherein the collection system comprises a collectionvessel configured to receive heating medium removed from the heatexchanger during the abnormal operation, and wherein the abnormaloperation comprises a cessation of flow or substantial reduction in aflow rate of the heating medium through the heat exchanger.
 13. Thewaste heat recovery unit of claim 12, wherein the abnormal operationcomprises a shutdown of the pump.
 14. The waste heat recovery unit ofclaim 12, wherein the abnormal operation comprises continued flow of theexhaust gas across the heat exchanger.
 15. The waste heat recovery unitof claim 12, wherein the collection system comprises a conduitconfigured to drain the heating medium from the heat exchanger to thecollection vessel.
 16. The waste heat recovery unit of claim 12,comprising a control system configured to detect the abnormal operation,and configured to automatically implement removal of the heating mediumfrom the coils during the abnormal operation.
 17. A method of operatinga waste heat recovery unit, the method comprising: circulating a heatingmedium through multiple tubes of a tube bundle in the waste heatrecovery unit and to users of the heating medium outside of the wasteheat recovery unit; passing a vapor over the tube bundle to transferheat from the vapor to the heating medium; detecting via a controlsystem an abnormal operation of the waste heat recovery unit, theabnormal operation comprising loss of circulation of the heating mediumthrough the multiple tubes; and removing the heating medium from themultiple tubes to a collection vessel in the waste heat recovery unitduring the abnormal operation to reduce or avoid thermal degradation ofthe heating medium.
 18. The method of claim 17, wherein the vaporcomprises exhaust gas.
 19. The method of claim 17, comprising dilutingthe vapor with air to reduce a temperature of the vapor during theabnormal operation.
 20. The method of claim 17, wherein removingcomprises draining the heating medium from the multiple tubes to thecollection vessel when the heating medium is a liquid and venting theheating medium from the multiple tubes to the collection vessel when theheating medium is a vapor.
 21. The method of claim 17, comprisingpurging the multiple tubes with an inert gas during the abnormaloperation.
 22. A method of constructing or retrofitting a waste heatrecovery unit (WHRU) to prevent or reduce thermal degradation of heatingmedium during abnormal operation of the WHRU, the method comprising:adding a conduit downstream of a tube bundle configured to transfer heatfrom an exhaust gas to the heating medium, wherein the conduit isconfigured to divert flow of the heating medium in the WHRU during theoff-specification operation of the WHRU, the abnormal operationcomprising cessation of normal flow of heating medium through the tubebundle; installing a collection vessel coupled to the conduit andconfigured to receive heating medium drained from the tube bundle viathe conduit during the abnormal operation; and installing a valve on theconduit, wherein the valve is configured to isolate the collectionvessel during normal operation of the WHRU and to place the collectionvessel in service during the abnormal operation.
 23. The method of claim22, comprising installing an inert-gas purge inlet configured to providean inert gas purge through tubes of the tube bundle during the abnormaloperation.
 24. The method of claim 22, comprising installing anotherconduit configured to receive vaporized heating medium from the tubebundle during the abnormal operation.
 25. The method of claim 22,comprising installing an ambient-air inlet on ductwork receiving theexhaust gas and housing the tube bundle.